The sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce audio signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be helped by the use of conventional hearing aids that amplify sound so that audio signals reach the cochlea and the hair cells. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous auditory prosthesis systems (e.g., cochlear implant systems) have been developed. Auditory prosthesis systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to stimulation sites (e.g., auditory nerve fibers) by way of one or more channels formed by an array of electrodes implanted in an auditory prosthesis patient. Direct stimulation of the stimulation sites leads to the perception of sound in the brain and at least partial restoration of hearing function.
Some conventional auditory prosthesis systems may be configured to use current steering to apply electrical stimulation to stimulation sites not directly associated with the electrodes implanted within an auditory prosthesis patient. For example, an auditory prosthesis system may concurrently stimulate multiple (e.g., two) electrodes that surround, but that are not directly associated with, a particular stimulation site in order to steer current to (and thereby apply electrical stimulation to) the stimulation site. One advantage of current steering is that the current used to concurrently stimulate the multiple electrodes is split between the multiple electrodes, thereby reducing the compliance voltage (i.e., the voltage maintained by the auditory prosthesis that governs a maximum level of stimulation current that can be delivered by the auditory prosthesis) required to generate the current. Unfortunately, however, even when current steering is used in a conventional auditory prosthesis system, the auditory prosthesis has to maintain a relatively high compliance voltage to account for situations in which the desired stimulation site is directly associated with a single electrode. In such situations, a relatively high compliance voltage is required because all of the current is applied to the single electrode.